Highly elastically or plastically extensible pressure-sensitive adhesive strips which can be detached again without residue or destruction by extensional stretching in the bond plane are known from, for example, U.S. Pat. No. 4,024,312 A, DE 33 31 016 C2, WO 92/01132 A1, WO 92/11333 A1, DE 42 22 849 A1, WO 95/06691 A1, DE 195 31 696 A1, DE 196 26 870 A1, DE 196 49 727 A1, DE 196 49 728 A1, DE 196 49 729 A1, DE 197 08 364 A1, DE 197 20 145 A1, DE 198 20 854 A1, WO 99/37729 A1, and DE 100 03 318 A1, and are referred to inter alia below as strippable pressure-sensitive adhesive strips.
Strippable self-adhesive tapes of this kind are frequently used in the form of single-sidedly or double-sidedly pressure-sensitive adhesive-sheet strips, which preferably have a nonadhesive grip region, from which the detachment operation is initiated.
Particular applications of such pressure-sensitive adhesive strips are found in DE 42 33 872 A1, DE 195 11 288 A1, U.S. Pat. No. 5,507,464 A, U.S. Pat. No. 5,672,402 A, and WO 94/21157 A1; specific embodiments are described in, for example, DE 44 28 587 A1, DE 44 31 914 A1, WO 97/07172 A1, DE 196 27 400 A1, WO 98/03601 A1, and DE 196 49 636 A1, DE 197 20 526 A1, DE 197 23 177 A1, DE 297 23 198 A1, DE 197 26 375 A1, DE 197 56 084 A1, DE 197 56 816 A1, DE 198 42 864 A1, DE 198 42 865 A1, WO 99/31193 A1, WO 99/37729 A1, and WO 99/63018 A1.
Fields of use of the aforementioned strippable pressure-sensitive adhesive strips include in particular the residuelessly and nondestructively redetachable fixing of light to moderately heavy articles in the home, workplace, and office segments. In these applications they replace conventional fastening means such as, for example, drawing pins, roundhead needles, thumb tacks, nails, screws, conventional self-adhesive tapes, and liquid adhesives. Key to the successful use of the pressure-sensitive adhesive strips is not only the possibility of residueless and nondestructive redetachment of bonded articles but also their quick and easy bonding and also their secure hold for the envisioned period of bonding. It should be borne in mind in particular here that the adhesive strips must function on a large number of substrates in order to be able to serve as a universal fixing in the home, workplace, and office segments.
Despite the fact that the literature cited above describes a broad range of pressure-sensitive adhesives for use in strippable self-adhesive tapes, commercial products currently on the market (for example, Tesa® Powerstrips® from tesa AG, 3M Command® adhesive strips from 3M, and Plastofix® Formule Force 1000 adhesive strips from Plasto S.A.) virtually without exception have pressure-sensitive adhesives based on styrene block copolymers with unsaturated polydiene blocks in the elastomer block.
These products based on the styrene block copolymers exhibit weaknesses in bond strength under the influence of atmospheric humidity or water. This behavior is particularly pronounced when bonds are made on hydrophilic substrates such as glass or ceramic. Particularly in the case of moisture exposure shortly after the bonding of moderately heavy articles, failure of the pressure-sensitive adhesive strips is a frequent occurrence. The holding power under the influence of moisture is reduced in particular in those pressure-sensitive adhesive strips which comprise nonpolar tackifier resins such as hydrocarbon resins or polyter-gene resins.
In the case of products comprising an intermediate foam carrier to which adhesive is applied on both sides, the reduction in bond strength under moisture exposure was more strongly pronounced than in the case of adhesive strips which are composed of just one layer of adhesive.
Failure of the bond occurs in the case of a pure peeling load and particularly in the case of a tipping/shearing load (where a torque is active, as in the case, for example, of the bonding of a hook with a particular lever on which something is hung) to a much greater extent than in the case of a pure shearing load.
DE 102 24 842 A1 describes an adhesive based on styrene block copolymers that by virtue of addition of super-absorbents possesses significantly improved holding power on hydrophilic substrates such as glass under moisture conditions. Nevertheless, the bonding performance in the case of long-lasting, very high atmospheric humidity, but especially under running water, is inadequate.
U.S. Pat. No. 6,569,521 B describes a redetachable adhesive strip with a strong, extensible carrier and an adhesive based on silicones, especially block copolymers of silicones and polyureas. This product can be removed from the substrate again by pulling in the bond plane; the bond strength to glass at 98% relative humidity is increased significantly in relation to that of adhesives based on styrene block copolymers. A disadvantage is that, owing to the stretching of the entire product, the only adhesives that can be used are those for which the bond strength is greatly reduced under extension.
For a large number of pressure-sensitive adhesives (PSAs), the bonding performance subsides considerably under the influence of high atmospheric humidity or water. Not only is it the case that damp or wet adhesive strips adhere much less well or even not at all, or that they are difficult to bond to wet substrates, but also existing bonds of an adhesive tape on a substrate can be impaired in their load-bearing capacity, or even fail completely, under the influence of humidity or water. This phenomenon is particularly pronounced in the case of hydrophilic substrates such as glass or ceramic products such as tiles. Ceramic tiles in particular are often found in bathrooms or kitchens, where for short periods the atmospheric humidity can rise very sharply. Hydrophilic substrates possess the property of often having a very thin layer of adsorbed water bound on the surface, which can be removed only at very high temperatures. As a result of this thin layer of water, it is very easy for moisture or water to be picked up by the glass. Owing to the molecular structure of the glass, it is even capable of taking water into the glass itself and not only of absorbing it on the surface. Similar considerations apply to ceramic products too.
If an adhesive tape is bonded to ceramic products or glass, a thin layer of water remains between the adhesive tape and the glass. This layer is so thin that the bonding properties of the adhesive tape are unaffected; the bond between adhesive tape and glass can be very strong, similar to that between steel and the same adhesive tape.
If moisture in the form of high atmospheric humidity or water acts on the bond, the layer of water between glass and adhesive strip may pick up further water, which causes the layer to grow. Water may also diffuse through the glass to the bond area. Consequently the bond performance is reduced to such an extent that it can lead to the bond failing.
In order to prevent this failure of the bond it is standard practice to use very soft adhesives which are able to close off all of the pores in the glass, so that the water is no longer able to diffuse into the intermediate layer. This route is taken, for example, in connection with the adhesive bonding of laminated glass sheets, where an isobutyl rubber adhesive is employed (according to Skeist, “Handbook of Adhesives”, 2nd edition, 1977).
An alternative to the very soft adhesives based on isobutyl rubbers are adhesives based on silicones or silicone-containing polymers. A disadvantage is that these adhesives frequently do not loose bond strength to a sufficient extent when stretched, to allow them to be removed from a surface.
In contrast to this, silicone compositions can frequently be peeled effectively from the substrate to which they have been bonded.
It is an object of the invention to develop a redetachable adhesive strip with adhesives that are formulated on a relatively broad formulating basis, said adhesive strip having a high bond strength even on hydrophilic substrates under high atmospheric humidity, and even under running water, but also being removable again without residue.